Nucleosynthesis Prescriptions Research Articles

Chemical evolution and nature of damped Lyman systems

We study the nature of damped Lyman α systems (DLAs) by means of a comparison between the observed abundances and models of chemical evolution of galaxies of different morphological type. In particular, we compare for the first time the abundance ratios as functions of metallicity and redshift with dust-corrected data. We have developed detailed models following the evolution of several chemical elements (H, D, He, C, N, O, Ne, Mg, Si, S, Fe, Ni and Zn) for elliptical, spiral and irregular galaxies. Each of the models is calibrated to reproduce the main features of a massive elliptical, the Milky Way and the Large Magellanic Cloud (LMC), respectively. In addition, we also run some models for dwarf irregular starburst galaxies. All the models share the same up to date nucleosynthesis prescriptions but differ in their star formation histories. The role of supernovae of different types (II, Ia) is studied in each galaxy model. Our main conclusions are as follows. (i) When dust depletion is taken into account, most of the claimed α/Fe overabundances disappear and DLAs show solar or subsolar abundance ratios. (ii) The majority of DLAs can be explained either by discs of spirals observed at large galactocentric distances or by irregular galaxies, such as the LMC, or by starburst dwarf irregulars observed at different times after the last burst of star formation. (iii) Elliptical galaxies cannot be DLA systems because they reach too high a metallicity at early times and their abundance ratios show overabundances of α-elements relative to Fe over a large range of [Fe/H]. (iv) The observed neutral gas cosmic evolution is compared with our predictions but no firm conclusions can be drawn in the light of the available data.

Oxygen, carbon and nitrogen evolution in galaxies

We discuss the evolution of oxygen, carbon and nitrogen in galaxies of different morphological type by adopting detailed chemical evolution models with different star formation histories (continuous star formation or starbursts). In all the models detailed nucleosynthesis prescriptions from supernovae of all types and low- and intermediate-mass stars are taken into account. We start by computing chemical evolution models for the Milky Way with different stellar nucleosynthesis prescriptions. Then, a comparison between model results and ‘key’ observational constraints allows us to choose the best set of stellar yields. Once the best set of yields is identified for the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals (in particular M101) and dwarf irregular galaxies. We compare our model predictions with the [C,N,O/Fe] versus [Fe/H], log(C/O) versus 12 + log(O/H), log(N/O) versus 12+ log(O/H) and [C/O] versus [Fe/H] relations observed in the solar vicinity and draw the following conclusions. (i) There is no need to invoke strong stellar winds in massive stars in order to explain the evolution of the C/O ratio, as often claimed in the literature. (ii) The predicted [O/Fe] ratio as a function of metallicity is in very good agreement with the most recent data available for the solar vicinity, especially for halo stars. This fact again suggests that the oxygen stellar yields in massive stars computed by either Woosley & Weaver or Thielemann, Nomoto & Hashimoto without taking into account mass loss, reproduce the observations well. (iii) We predict that the gap observed in the [Fe/O] versus [O/H] at [O/H] ∼− 0.3 dex should be observed also in C/O versus O/H. The existence of such a gap is predicted by our model for the Milky Way and is caused by a halt in the star formation between the end of the thick disc and the beginning of the thin disc phase. Such a halt is produced by the adopted threshold gas density for the star formation rate. (iv) This threshold is also responsible for the prediction of a very slow chemical enrichment between the time of formation of the solar system (4.5 Gyr ago) and the present time, in agreement with new abundance measurements. (v) The chemical evolution models for dwarf irregulars and spirals, adopting the same nucleosynthesis prescriptions of the best model for the solar neighbourhood, well reproduce the available constraints for these objects. (vi) By taking into account the results obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and DLAs) we conclude that there is no need for claiming a strong primary component of N produced in massive stars (M > 10 M� ). (vii) Moreover, there is a strong indication that C and N are mainly produced in low- and intermediate-mass stars, at variance with recent suggestions that most of the C should come from massive stars. In particular, intermediate-mass stars with masses between 4 and 8 M� contribute mostly to N (both primary and secondary) whereas those with masses between 1 and

Abundance Gradients and the Formation of the Milky Way

In this paper we adopt a chemical evolution model, which is an improved version of the Chiappini, Matteucci, & Gratton model, assuming two main accretion episodes for the formation of the Galaxy, the first forming the halo and bulge in a short timescale and the second one forming the thin disk, with a timescale that is an increasing function of the Galactocentric distance (being of the order of 7 Gyrs at the solar neighborhood). The present model takes into account in more detail than previously the halo density distribution and explores the effects of a threshold density in the star formation process during both the halo and disk phases. The model also includes the most recent nucleosynthesis prescriptions concerning supernovae of all types, novae, and single stars dying as white dwarfs. In the comparison between model predictions and available data, we have focused our attention on abundance gradients as well as gas, stellar, and star formation rate distributions along the disk, since this kind of model has already proven to be quite successful in reproducing the solar neighborhood characteristics. We suggest that the mechanism for the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially contribute to building up the outer disk. We also show that the existence of a threshold density for the star formation rate, both in the halo and disk phase, is necessary to reproduce the majority of observational data in the solar vicinity and in the whole disk. In particular, a threshold in the star formation implies the occurrence of a gap in the star formation at the halo-disk transition phase, in agreement with recent data. We conclude that a relatively short halo formation timescale (0.8 Gyr), in agreement with recent estimates for the age differences among Galactic globular clusters, coupled with an inside-out formation of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents, at the moment, the most likely explanation for the formation of the Milky Way. This scenario allows us to predict abundance gradients and other radial properties of the Galactic disk in very good agreement with observations. Moreover, as a consequence of the adopted inside-out scenario for the disk, we predict that the abundance gradients along the Galactic disk must have increased with time and that the average ratios in stars (halo plus disk) slightly decrease going from 4 to 10 kpcs from the Galactic center. We also show that the same ratios increase substantially toward the outermost disk regions and the expected scatter in the stellar ages decreases, because the outermost regions are dominated by halo stars. More observations at large Galactocentric distances are needed to test these predictions.

The Chemical Evolution of the Galaxy: The Two‐Infall Model

We present a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin), but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin disk is much longer than that of the thick disk, implying that the infalling gas forming the thin disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin disk is assumed to be a function of Galactocentric distance, leading to an inside-out picture for the Galaxy's building. The model takes into account the most up-to-date nucleosynthesis prescriptions and adopts a threshold in the star formation process, which naturally produces a hiatus in the star formation rate at the end of the thick-disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighborhood and for the whole disk. Among these constraints, the tightest is the metallicity distribution of the G-dwarf stars, for which new data are now available. Our model fits these new data very well. The model also predicts the evolution of the gas mass, the star formation rate, the supernova rates, and the abundances of 16 chemical elements as functions of time and Galactocentric distance. We show that, in order to reproduce most of these constraints, a timescale of ≤1 Gyr for the (halo) thick disk and of 8 Gyr for the thin disk's formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk (R < 1 R☉) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner gradients steepen during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process are discussed.

3He in Planetary Nebulae: A Challenge to Stellar Evolution Models

The discrepancy between the observed abundances of 3He in the ISM and those predicted by stellar and galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (PNe) with the corresponding predictions of stellar evolution theory. The determination of the mass of the PNe progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of 3He in the envelope of all low-mass stars is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the galactic evolution of 3He with the help of a series of models with standard and non-standard nucleosynthesis prescriptions. The results are found to be consistent with the observed galactic abundances only if the vast majority of low-mass stars follows non-standard prescriptions. This implies that either the sample of PNe nebulae under exam is highly biased, or the solution to the 3He problem lies elsewhere.

Roles of SN Ia and SN II in ICM Enrichment

Based on ASCA observations Mushotzky et al. (1996, ApJ 466, 686) have recently derived the relative abundance ratios of α-elements to iron, [α/Fe] ≃ 0.2–0.3, for four rich clusters, and have suggested that the origin of metals in an intra-cluster medium (ICM) is not a type-Ia supernovae (SNIa), but a type-II supernovae (SNII). However, these authors used the solar photospheric iron abundance for ASCA data reduction, while the meteoritic iron abundance is usually adopted in chemical-evolution studies. It is true that although the photospheric and meteoritic solar abundances are consistent for most of the elements, a serious discrepancy is known to exist for iron; indeed, the photospheric abundance of iron is NFe/NH = 4.68 · 10−5 by number, while the meteoritic value is 3.24 · 10−5. The argument concerning the relative roles of SNIa and SNII in ICM enrichment is quite sensitive to the precise values of [α/Fe], and one should use an identical solar iron abundance in data reduction as well as in theoretical arguments. We therefore adopt the meteoritic iron abundance, which is consistent with chemical-evolution studies, and shift Mushotzky et al.'s ASCA data by Δ[α/Fe] ≃ −0.16 dex. By comparing the corrected [α/Fe] values with theoretical nucleosynthesis prescriptions of SNIa and SNII, we reach a conclusion that an SNIa iron contribution of 50% or higher in the ICM enrichment could not be ruled out, and might indeed be favored based on the ASCA spectra.

The early nucleosynthesis of O and Fe

The new observational [O/Fe] vs [Fe/H] relationship in halo dwarfs is analyzed in terms of a model of chemical evolution for the solar neighbourhood. The role of nucleosynthesis prescriptions for SN II and SN I (both Ia and Ib) is analyzed. We show that by adopting iron yields for SN II according to the observations of SN 1987A and by assuming double degenerate systems to be the progenitors of SN Ia and Wolf-Rayet stars to be the progenitors of SN Ib, it is possible to reproduce quite well the behaviour of the [O/Fe] ratio for halo and disk stars. Nevertheless, best agreement is obtained when introducing in addition a variable IMF during the halo evolution.